One method of manufacturing ophthalmic lenses is by cast molding. In a cast molding process a lens precursor material is introduced into a lens-shaped cavity formed between lens mold portions. The lens precursor material is then cured and/or polymerized, thereby forming an ophthalmic lens. It is then necessary to remove the lens from the mold assembly. First the two mold portions are separated (known as demolding). Typically, following demolding, the lens remains in contact with one of the lens mold portions, often due, at least in part, to the material properties of the lens and the lens mold. Consequently, the next step in the cast molding process involves removing the lens from the mold portion (known as delensing).
Lenses at this stage of the manufacturing process are easily damaged, being susceptible to surface damage and tearing which renders the lens unusable. Moreover, the optical performance of a lens is strongly dependent on its shape. Therefore, any stretching of the lens during the demolding sequence may affect the optical performance of the lens, and therefore impact the ability of the lens to correct vision. Thus, in order to be effective, any manufacturing method must minimize the risk of damage to, or deformation of, the lens during removal from the lens mold portion.
As well as being easily damaged, the lens may be very flexible, i.e., the lens may have a very low elastic modulus. This is particularly true in the case of silicon hydrogel contact lenses but also applies to other ophthalmic lenses. In the case of a highly flexible lens, deformation of the mold carrying the lens may simply result in deformation of the lens, rather than successful delensing, because the lens does not favor its natural shape in the same way as a more rigid lens. Consequently, a more radical deformation may be required to detach the lens from the mold, thereby increasing the risk of damage to the lens or failure of the mold.
Methods for removing a lens from a mold portion may be categorized as mechanical methods and non-mechanical methods.
One known non-mechanical method of removing a lens from a mold portion is to soak the lens and the mold portion in a liquid solution such that the lens floats off of the mold. However, once hydrated or otherwise swollen with liquid, the lens is much more difficult to handle, which complicates downstream manufacturing processes. Such methods also significantly increase production time because of the length of time required for the lens to absorb the necessary amount of liquid.
U.S. Pat. No. 7,811,483 describes mechanical methods and systems for delensing ophthalmic lenses by using gas. A mold portion carrying a lens is radially compressed, and a gas, such as air, is directed towards the lens. The radial compression of the mold portion provides a void between the lens and the mold portion near the edge of the lens, which allows the gas to lift the edge of the lens. However, deforming the mold from the side can cause the lens to come off of the mold asymmetrically, resulting in difficulties in downstream manufacturing processes and increased risk of damage.
U.S. Pat. No. 8,105,070 also discloses the use of gas jets to lift the edge of a lens carried by a mold portion. This document describes applying a force in a first direction to diametrically opposed portions of the outer flanges of the mold while applying a force in a second, opposite direction, to the center of the mold, such that the mold is peeled away from the edges of the lens and can then be lifted by the gas jets.
U.S. Pat. No. 8,038,912 describes a method of delensing a lens by locating a circular shear ring against the lens-side surface of the mold. A plunger is then pressed against the back surface of the mold such that the mold is pressed between the shear ring and the plunger. The diameter of the plunger is less than that of the shear ring so the edge of the mold is peeled away from the lens as the plunger is pressed against the back of the mold.
The methods described above can be used to remove lenses from mold portions. However, severe deformation of the mold over a limited radius, as required in the above methods, may result in fracturing of the mold, thereby contaminating the lens and rendering it unusable. The degree of deformation also increases the risk of defects being produced on the lens surface. Consequently, it is advantageous to provide improved methods of removing an ophthalmic lens from a mold portion, which result in a lower rate of rejection of lenses because of damage caused by the delensing process.